In recent mobile communication devices (mobile phone or a like), from a viewpoint of miniaturization and integration of a signal receiving device, a direct conversion-type signal receiving device is developed and proposed. FIG. 12 is a diagram showing a configuration made up of an RF (Radio Frequency) front end having a multistage gradual declining filtering architecture disclosed in related art Patent Reference 1.
As shown in FIG. 12, an RF signal inputted from a receiving signal input terminal 1201 is amplified by an LNA (Low Noise Amplifier) 1202 and is inputted to a mixer 1203. By using a local oscillator output signal fed through a buffer 1221 from a local oscillator 1204, an RF frequency of a receiving signal is down-converted into a 1F frequency (one fifth of the RF frequency). An output signal from the mixer 1203 is inputted to mixers 1261 and 1262. The mixers 1261 and 1262 make up an I/Q mixer in which the mixers 1261 and 1262 serve as an I mixer and Q mixer respectively. In some cases, there is provided an intermediate frequency amplifier (IF amplifier) between the mixer 1203 and I/Q quadrature mixer).
A local oscillation output signal from the local oscillator 1204 is inputted to a 4 frequency divider 1205. The 4 frequency divider 1205 divides a frequency of a local oscillation output signal by 4. The frequency-divided frequency divider signal is supplied, as a second local oscillation output signal, to the mixers 1261 and 1262 through buffers 1222 and 1223. However, in the case of the RF front end disclosed in the above Patent Reference 1, the demodulation of in-phase (I) and quadrature (Q) signal using the RF signal requires not only the mixers 1261 and 1262 for the I signal and Q signal but also the buffers 1222 and 1223, filters (LPFs) 1271 and 1272, and A/D (Analog to Digital) converters 1281 and 1282 for the I and Q signals.
Moreover, a direct conversion-type signal receiving device is disclosed in related art Patent Reference 2 in which only one mixer is used for demodulation of I and Q signals using an RF signal. FIG. 13 is a diagram showing the direct conversion-type signal receiving device. In FIG. 13, a radio modulated signal received by an antenna 1301 passes through a band pass filter (BPF) 1302, a low noise amplifier (LNA) 1303, and a band pass filter 1304 sequentially and is then inputted to a mixer 1305. Here, a quadrature converting section involving the mixer 1305 is operated according to the direct conversion method and a local oscillation signal to be supplied to the mixer 1305 has the same frequency as that of a receiving signal. The local oscillation signal to be supplied to the mixer 1305 is then supplied through either of an in-phase (I) path 1371 or a quadrature (Q) path 1373. Which pass is to be used is selected according to a phase pass control signal 151 fed from a timing signal generating unit 1315.
When the phase pass control signal 151 is “H”, the mixer 1305 and a local oscillator 1375 are connected to each other by a phase pass on/off switch 1374 through the in-phase pass 1371, that is, not through a −90° phase shifter 1372. On the other hand, when the phase pass control signal 151 is “L”, the mixer 1305 and the local oscillator 1375 are connected to each other by the phase pass on/off switch 1374 through the quadrature pass 1373, that is, through the −90° phase shifter 1372.
An output terminal of the mixer 1305 is connected to an A/D converter 1311 through a first low pass filter (LPF) 1361 and a second low pass filter 1362 making up a filter section, a filter on/off switch 1308 making up a filter switching section, a variable gain amplifier (VGA) making up an amplifier section. The filter on/off switch 1308, when an IQ pass control signal 152 from the timing signal generating section 1315 is “H”, selects the first LPF 1361 and, when the IQ pass control signal 152 is “L”, selects the second LPF 1362. An output terminal of the A/D converter 1311 is connected to an in-phase pass output 131 by an IQ output pass on/off switch 1312 when the IQ pass control signal 152 is “H” and is connected, when the IQ pass control signal 152 is “L”, to a quadrature pass output 132.
The direct conversion signal receiving device disclosed in the related art Patent Reference 2 has only one of each of the mixer 1305, a buffer amplifier located between the local oscillator and the mixer, A/D converter 1311, and the like and, therefore, when compared with the signal receiving device of the direct conversion type disclosed in the related art Patent Reference 1 in which two of these components are provided for the in-phase and quadrature signals, an area and power consumption can be reduced. Moreover, an influence caused by variations in functions of components prepared for the in-phase and quadrature signals.
Further, another direct conversion-type signal receiving device is also disclosed in related art Patent Reference 3. The disclosed signal receiving device is so configured to convert an output signal fed from a local oscillator into one pair of inputted signals having a quadrature phase relation with each other, in a digital phase shifter/frequency divider, and is inputted to one pair of mixers. Furthermore, the related art Patent Reference 4 discloses a frequency synthesizer. The disclosed frequency synthesizer includes a frequency divider having a phase judging unit to judge a phase of a frequency divider output signal in which a phase of the frequency divider output signal is judged by a phase judging unit and an oscillation frequency of a voltage control oscillator is controlled.